The objective of this project is to test and refine a device to assist a surgeon in locating a target of interest as defined from pre-operative medical images. The method, an outgrowth of stereotactic surgery, combines recent advances in medical imaging, high precision position sensing, robotics and computer technology to provide the surgeon with an interactive display of the present position of a surgical instrument on the pre- operative images. Once the device has been calibrated and its accuracy defined and validated, the device can be used to: 1) guide the surgeon directly to the target point, minimizing the size of the total resection, 2) guide the surgeon to objects, such as tumor margins, which may be defined on CT or MR images but be visually indistinguishable from normal tissue, 3) guide the placement of brachytherapy radiation sources for optimum dose distribution, 4) steer laser resection and 5), guide electrode and biosensor placement. If successful, the arm should incorporate the stereotactic advantage of accurate registration of image and physical space without the requirements of present frame-based systems.